I don't ride my horse onto their property to ask. Clean yourself up, put on some decent clothes (not dressy, but clean and well fitting) and drive your car down to ask if it's ok.
Tell them who wants to ride, your best bet is just yourself, most landowners don't want randoms coming on their property. At first ask for a specific date/time, you have a much better chance of a onetime yes than a blanket yes. At least at first, after the landowner gets to know you a little better they may just offer for you to ride whenever you'd like.
To up your chances of getting free rein to ride whenever you'd like be respectful of their property. Don't ask to ride when it's wet, clean up after yourself and your horse (yes this includes manure if its on a yard or driveway), be SAFE (landowners worry about liability a lot), if they have small children offer a pony ride. Basically be nice, respectful, and develop a relationship with them.